This invention relates to electrical multi-channel signal transmission systems, in which different signals are transmitted by carriers (i.e. carrier waves) with different frequencies.
In practice, when transmitting such signals from one place to another, whether through the atmosphere and/or along wires, it is usually necessary to pass the carriers, with their signals, through non-linear electrical devices, which cause intermodulation products (often known as beats) to be produced by interaction between the carriers. Such non-linear electrical devices may for example be amplifiers. These intermodulation products may interfere with the signals carried by other carriers, thereby rendering clear reception of the signals impossible.
One well known example of a transmission system in which such problems arise is in the system known as cable television, in which television programs being transmitted through the atmosphere are picked up by a sophisticated antenna at a central station and, after suitable processing including amplification, are transmitted by cable to individual subscribers. The advantage of such a system is that the subscribers can receive television programs from places which are too far away from the subscriber to enable the programs to be picked up by a simple antenna of the kind normally found on a domestic television receiver. Processing of the signals and their carriers at the central station, and perhaps also at sub-stations, includes their treatment by non-linear electrical devices, such as certain kinds of amplifiers, which cause intermodulation products to be produced, with consequent likelihood of interference, as previously mentioned.
The problem could be avoided if the frequencies of the various carrier waves were allocated in such a manner that the intermodulation products were not of such frequencies as to produce unwanted interference with other carrier frequencies. For example, if each carrier frequency was an integral multiple of the difference in frequency between each carrier in the series, the significant intermodulation products would be at the frequencies of other carriers, and hence would not be detected as modulation by the signal reception equipment. This is known as a harmonically related carrier system. However, in many existing transmission systems of this type, for example systems for transmitting radio and television programs, the carrier frequencies have been established for many years, and it is not practically possible to change the carrier frequencies at the original transmitting station.
It has been previously proposed to avoid the problem found with existing transmission systems by receiving the carriers, and of course the signals carried thereby, on their original frequencies at a central station, and re-transmitting the signals on harmonically related carriers, which are obtained from a common frequency source. However, certain difficulties are imposed by this method. For example, domestic television receivers can only be tuned to certain selected channels, that is to say to only receive carriers and their signals at certain selected frequencies, since they only have a main tuner with pre-set positions corresponding to the frequencies of the carriers that the television set is intended to receive, and it is not practically possible to manufacture a main tuner with pre-set positions which correspond exactly to the carrier frequencies, since in practice such frequencies vary from time to time about a fixed value. The receivers are therefore also provided with a fine tuner, which can be adjusted to provide fine tuning when the main tuner has been adjusted to a pre-set position. The main tuner is set for adjustment to the frequencies at which the various carriers are originally transmitted, such frequencies having been established many years ago, as previously mentioned. If the signals, that is to say the television programs, are re-transmitted on harmonically related carriers, it is found that it is not possible to satisfactorily tune all domestic television receivers to the new carrier frequencies, because the new carrier frequencies are so far apart from the original carrier frequencies, for which the main tuner is pre-set, that the fine tuner does not have a sufficiently wide range of adjustment to enable the new carrier frequencies to be received.
Another way of overcoming the problem would be to eliminate non-linear electrical devices, such as single-ended amplifiers, i.e. non push-pull amplifiers, with which many existing cable television systems are equipped. Though it is possible to reduce the problem by replacing the single-ended amplifiers by amplifiers of the push-pull kind, the cost of such a procedure is undesirably high.